Stopping of molecules and clusters

The interaction of swift cluster and molecular ions with solids has been studied with increasing interest in the past decades. Experiments have produced valuable information on cluster energy losses, screening effects, charge states, and dynamical interactions among correlated ions, which in many cases have not been obtained using single atomic ions. These studies show differences (vicinage effects) in the energy loss per nucleon and also ill charge equilibrium conditions both for small and large clusters Theoretical studies dealing with vicinage effects in ion cluster interactions, using the most basic models, have provided a semi-quantitative view of cluster energy losses which partially agree with experiments, but still leave several questions open. In particular, the dielectric models predict a diminished stopping power in the low-velocity range, as a result of negative interferences, whereas at high velocities a significant enhancement of the energy loss is expected. The behavior at high velocities has been confirmed experimentally using small molecular ions. In addition, there is clear experimental evidence of diminished stopping values at low velocities, but still the possible influence of various mechanisms should be clarified, like the role of charge state equilibrium, alignment, and non-linear quantum effects. Modifications in charge state equilibrium may be specially important in the case of large clusters. The purpose of this work is, on one side, to review the current knowledge and recent progress in this field, and, on the other, to reformulate the theory of cluster energy loss in order to incorporate the effects of charge equilibrium according to the most recent experimental evidences. (C) 2000 Elsevier Science B.V. All rights reserved.